PROJECT SUMMARY In the mitral valve (MV) several pathological factors such as mitral regurgitation (MR) and degenerative myxomatous mitral valve disease (MMVD), have been shown to affect tissue structure and composition. Current clinical treatments for both functional and degenerative MR include ring annuloplasty. Though beneficial in the short-term, it has been shown to be less promising in the long term, with repair failure as high as 60%. Mechanical stress is a strong etiological factor: alterations in mechanical loading caused by surgical repair lead to stress-induced changes in valve interstitial cell (VIC) function that affect both tissue structure and composition, ultimately leading to repair failure. The link between tissue-level stresses and cellular homeostatic response is essential in understanding valve disease progression and in developing novel approaches to improve surgical repair. We thus hypothesize that mitral VIC (MVIC) deformation is a major driver for cellular mechanoregulation, and that abnormal mechanical stimuli lead to non-physiological MVIC deformations that result in phenotypic activation and altered biosynthetic activity. Extending these ideas for pathological valve conditions, we further hypothesize that myxomatous MVICs can exhibit signs of phenotypic reversal after being exposed to a physiological strain regime in a healthy microenvironment. Exploiting this knowledge can lead to improved surgical repair techniques for both functional and degenerative MR. We will address our hypotheses with the following two aims: (1) Investigate the response of normal MVICs to a range of physiological and non-physiological biaxial loading conditions. First, using intact ovine MV anterior leaflets. Then, by utilizing a human MVIC seeded engineered-tissue approach to characterize the responses of normal human MVICs to biaxial loading conditions. (2) Characterize the response of human myxomatous MVICs to physiological and non-physiological biaxial loading conditions. Building upon our hypothesis that MVIC deformation drives homeostatic response, we will use our established MV macro-micro finite element model to simulate in vivo MVIC deformation under different surgical repair scenarios and bracket physiological and non-physiological regions within the range of in vitro biaxial loading conditions used in (1). We will then explore the potential to induce phenotypic reversal in myxomatous MVICs after an in vitro treatment in an optimal mechano-microenvironment.